Method and apparatus for high-pressure wafer processing and drying

ABSTRACT

A system for high-pressure drying of semiconductor wafers includes the insertion of a wafer into an open vessel, the immersion of the wafer in a liquid, pressure-sealing of the vessel, pressurization of the vessel with an inert gas, and then the controlled draining of the liquid using a moveable drain that extracts water from a depth maintained just below the gas-liquid interface. Thereafter, the pressure may be reduced in the vessel and the dry and clean wafer may be removed. The high pressure suppresses the boiling point of liquids, thus allowing higher temperatures to be used to optimize reactivity. Megasonic waves are used with pressurized fluid to enhance cleaning performance. Supercritical substances are provided in a sealed vessel containing a wafer to promote cleaning and other treatment.

This application is a continuation of application Ser. No. 09/481,651,filed Jan. 12, 2000 and now U.S. Pat. No. 6,286,231.

FIELD OF THE INVENTION

The present invention relates to the processing and drying ofsemiconductor wafers or similar items at high pressures.

BACKGROUND OF THE INVENTION

Wet chemical processes are a crucial part of semiconductor devicefabrication. Such processes include etching of films, removal ofphotoresist, and surface cleaning. Over the years, specific applicationshave spawned the development of numerous chemistries for wet processing,including APM (a mixture of ammonium hydroxide, hydrogen peroxide, andwater), HPM (hydrochloric acid, hydrogen peroxide, and water); SPM(sulfuric acid and hydrogen peroxide), SOM (sulfuric acid and ozone),and others for specific cleaning or etching tasks. Many of thesechemistries are used at or near their boiling points, since chemicalreactivity, and therefore the effectiveness of the cleaning, is afunction of temperature. Recent developments in wet processingtechnology have incorporated the use of various gases with aqueous orother liquid solutions to accomplish a desired process objective. Forexample, the use of ozone and water creates a strong oxidizing solutionthat may be useful in semiconductor processing. The use of hydrochloricacid or ammonia gas injected into water to create a low or high pHsolution with specific properties are additional examples of the use ofgas technology.

The use of gas/liquid process mixtures is often limited by gassolubility and temperature constraints. Solubility limitations areheightened when aqueous solutions are used. The limited solubility ofgases such as ozone in water at ambient conditions, for example, limitsthe effectiveness of ozone/water solutions for oxidizing organiccompounds, as there is simply not enough ozone available to promote theoxidation process. Reactivity constraints related to temperature areoften intertwined with solubility limitations. For example, thesolubility of virtually all gases in liquid solution decreases withincreases in temperature. Chemical reactivity, however, increases withincreasing temperature. These two factors are in conflict with eachother for process optimization. Additionally, many of the aqueoussolutions used in semiconductor processing are limited by their boilingpoints. One reason it is desirable to avoid boiling is to preventcavitation and suppress bubble formation for more effective use ofmegasonic waves in cleaning wafer surfaces. For example, a 5:1:1 mixtureof water, ammonium hydroxide, and hydrogen peroxide will boil atapproximately 65 C. Accordingly, such a mixture cannot be maintained inliquid form at elevated temperature unless the composition is changed toelevate the boiling point.

A critical step in the wet-processing of semiconductor device wafers isthe drying of the wafers. Any rinsing fluid that remains on the surfaceof a semiconductor wafer has at least some potential for depositingresidue or contaminants that may interfere with subsequent operations orcause defects in the end product electronic device. In practice,deionized (“DI”) water is most frequently used as the rinsing fluid.Like most other liquids, DI water will “cling” to wafer surfaces insheets or droplets due to surface tension following rinsing. An idealdrying process would operate quickly to effect the removal of thesesheets or droplets and leave absolutely no contaminants on the wafersurfaces, while presenting no environmental or safety risks.

Although various technologies have been used to dry wafers and reducethe level of contaminants left on the wafer surface after drying, themost attractive technology currently available falls under the broadcategory of surface tension trying. A typical surface tension dryersaccomplishes wafer drying using the following steps: (1) wafers areimmersed in a rinse medium; (2) the rinse medium is either drained awayfrom the wafers or the wafers are lifted out of the rinse medium,exposing them to a displacement medium that is typically an inertcarrier gas containing a percentage of organic vapor, usually analcohol, such as isopropyl alcohol (“IPA”); (3) the organic vapordissolves in the surface film of the rinse medium, creating aconcentration gradient in the liquid, which in turn creates a surfacetension gradient that enables the higher surface tension in the bulkliquid to essentially “pull” the lower surface tension liquid away fromthe wafer surface along with any entrained contaminants to yield a drywafer; and, in some instances, (4) the displacement medium may be purgedfrom the locale of the wafer using a drying medium such as an inert gasstream. Additionally, the carrier gas may be heated to assist in dryingand to prevent liquid condensate from forming on the wafer surfaces.

Conventional surface tension drying technology is limited by at leastthe following factors: (1) it involves the inherent hazard of causingIPA, a flammable liquid, to be boiled at a temperature well in excess ofits flash point; (2) it requires the consumption of IPA at relativelyhigh rate; and (3) it creates relatively high fugitive organic vaporemissions.

In light of the limitations inherent to these and other processing anddrying technologies, it is an object of one aspect of the presentinvention to suppress the boiling point of a wafer processing liquid topermit processing at elevated temperatures.

It is an object of another aspect of the present invention to increasethe solubility of gases in the liquid phase to enhance chemicalreactivity.

It is yet another object of the present invention to prevent cavitationand suppress bubble formation for more effective use of megasonic wavesto enhance cleaning performance.

It is still another object of the present invention to reduce oreliminate the need for using an organic vapor as a drying ordisplacement medium in a wafer drying process.

The term “wafer” means a semiconductor wafer, or similar flat media suchas photomasks, optical, glass, and magnetic disks, flat panels, etc.

SUMMARY OF THE INVENTION

To these ends, in a first aspect of the invention, a method of drying awafer includes placing a wafer into a vessel, immersing the wafer in aliquid, pressure-sealing the vessel, pressurizing the vessel, and thencontrolling removal of the liquid. Thereafter, the pressure may bereduced in the vessel and the dry and clean wafer may be removed.

The drying process operates at a pressure preferably between 10 and 100atmospheres, and more preferably, between 20 and 50 atmospheres. The gasdelivered to the vessel is advantageously also temperature controlled.The high pressure promotes the dissolution of gas into the liquid,generating a concentration gradient and a related surface tensiongradient. As the liquid along the surface is drained away to exposefresh liquid to the gas, the gas preferably continues to be supplied tomaintain the surface tension gradient. The surface tension gradientpulls liquid from the surface of the wafer as the liquid level descends,yielding a clean, dry wafer.

In second aspect of the invention, in a method for processing a wafer,high pressure is used to raise the liquid boiling point allowingprocessing at higher temperatures, to increase reactivity. The methodmay advantageously use a variety of liquids and gases to achievespecific objectives.

In a third aspect of the invention, megasonic waves are used inconjunction with pressurized fluid to yield enhanced cleaningperformance with higher efficiency.

In a fourth aspect of the invention, supercritical substances areprovided in a sealed vessel containing a wafer to promote cleaning andother treatment.

In a fifth aspect of the invention, an apparatus for processing wafersat high pressures is provided. Preferably, the apparatus includes apressure sealable vessel, a floating or hinged moveable drain within thevessel, and orifices for adding liquid and gas to the vessel.

In a sixth aspect of the invention, phase changes between liquid phaseand critical phase are used to process a wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparentfrom the following detailed description and drawings, which discloseembodiments of the invention. It should be understood, however, that thedrawings are designed for the purpose of illustration only and are notintended as a definition of the limits of the invention.

In the drawings, where the same reference characters denote the sameelements, throughout the several views:

FIG. 1 is a flow diagram illustrating a high pressure wafer drying;

FIG. 2 is a schematic drawing of a high pressure wafer drying system forperforming the drying process shown in FIG. 1;

FIG. 3 is a flow diagram illustrating a high pressure wafer processingmethod;

FIG. 4 is a schematic drawing of a high pressure wafer processing systemfor carrying out the processing steps shown in FIG. 3;

FIG. 5 is a flow diagram illustrating a high pressure megasonic waferprocessing;

FIG. 6 is a diagram illustrating the processing steps of a supercriticalwafer processing method of the present invention;

FIG. 7A is a schematic, cross-sectional, side view of a wafer processingapparatus of the present invention;

FIG. 7B is a schematic, cross-sectional, plan view of the apparatus ofFIG. 7A along section line “A—A” in FIG. 7A;

FIG. 8A is a schematic, cross-sectional, side view of a firstalternative wafer processing apparatus of the present invention;

FIG. 8B is a schematic, cross-sectional, plan view of the apparatus ofFIG. 8A along section line “B—B” in FIG. 8A;

FIG. 9A is a schematic, cross-sectional, side view of a secondalternative wafer processing apparatus of the present invention; and

FIG. 9B is a schematic, cross-sectional, plan view of the apparatus ofFIG. 9A along section line “C—C” in FIG. 9A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the basic steps of a high pressure wafer dryingmethod in accordance with one aspect of the present invention.

Referring now in detail to FIG. 1, a wafer or batch of wafers is placedinto a vessel, as represented by step 10. Liquid is delivered into thevessel to create a liquid level, as represented by step 12. Deionizedwater is a preferred process liquid, since it is inexpensive,non-reactive with the wafer material, and presents no vapor emissionproblems. Other liquids, including water-based mixtures, may be usedinstead. Preferably the liquid immerses the wafer completely so that theliquid level is above the highest point of the wafer. Further, theliquid level preferably overflows at least one wall of the vessel so asto flush away any contaminants from inside the vessel or from thesurface of the wafer. The vessel is then closed with a pressure-tightlid to contain elevated pressures within the vessel, as represented bystep 14.

Once the vessel is closed and pressure-sealed, pressurized gas isdelivered into the vessel through an orifice located high enough in thevessel so that it is not submerged by the liquid within the vessel, asrepresented by step 16. The gas preferably is inert with significantsolubility in the liquid. Carbon dioxide or argon are examples of gasesthat may be advantageously used with deionized water within the vessel,since these inert gases have relatively high solubility in water. Thecontinued delivery of pressurized gas into the sealed vessel elevatesthe pressure within the vessel. Since the solubility of virtually allgases in liquids increases with pressure, the elevated pressure withinthe vessel increases the dissolved gas concentration at the surface ofthe liquid. This creates a surface tension gradient.

As represented by step 18, the liquid then starts to be drained out ofthe vessel, such as by the opening of a throttle valve. The drainingoccurs via the elevated pressure within the vessel forcing the liquidthrough the valve to a lower pressure region. As further represented bystep 18, it is important to perform this draining from an extractiondepth maintained just below the liquid level within the vessel. This isimportant for two reasons. The exit point just below the liquid surfaceallows the surface film to be constantly drained off, resulting in afresh layer for the gas to dissolve into, thereby replenishing thesurface tension gradient. In addition, the slightly submerged exit pointseals the gas from venting directly out of the vessel. The gas isprevented from escaping through the path of least resistance, therebyincreasing the amount of gas which dissolves into the liquid surface.

The delivery rate of gas to the vessel must be adequate to create areasonable liquid drain rate. The preferred operating pressure range isbetween 10 and 100 atmospheres, more preferably between 20 and 50atmospheres. At these operating pressures, a flow rate of between 1 and10 liters per minute is adequate to provide a sufficient surface tensiongradient at the liquid-gas interface within a 40 liter pressurizedprocess vessel. In addition to the gradient at the top surface of thebulk liquid, there is also a horizontal gradient across the top surfaceresulting from the draining off of liquid from the sides.

As the liquid drains, the level within the vessel drops and the surfacetension gradient pulls the liquid from the wafer surface. This drainingcontinues until the liquid level drops completely below the dried wafer.As represented in step 20, the pressure within the vessel is reduced toambient conditions, such as by reducing gas delivery to the vessel.Then, the vessel can be opened and the dry wafer removed, as representedin step 22.

The gas delivery orifice is located so that it is not submerged by theliquid, as represented in step 16. This prevents the injection of gasinto the liquid, which could cause bubbling or droplet carryover.Bubbles within the liquid are detrimental to the performance of thedrying process, since such bubbles bursting near the wafer surface couldcause water spots on any portion of the wafer surface previously dried,and cause contaminants to adhere to the wafer surface.

FIG. 2 is a schematic diagram of a high pressure wafer drying system forcarrying out the drying steps illustrated in FIG. 1. At least one wafer50 is placed within a process vessel 52 suitable for containing elevatedpressures. The process vessel 52, which is preferably emptied of anyliquid before the wafer 50 is placed inside so as to flush away anycontaminants, is then closed around the wafer 50. The process vessel 52preferably contains a moveable drain 54 that maintains a extractiondepth just below the liquid-gas interface 53 within the vessel 52, suchas by floating on the surface of the liquid 56 within the vessel 52 anddraining liquid 56 from the bottom 55 of the moveable drain 54. Once theprocess vessel 52 is closed, a liquid supply valve 64 opens to allowliquid to flow from a liquid supply 60, through a liquid filter 62, andinto the vessel 52. The liquid supplied to the vessel 52 may beoptionally temperature controlled by a liquid supply heat exchanger 66and a liquid supply temperature sensor 68 located downstream of theliquid supply heat exchanger 66. Similarly, the vessel itself may beoptionally temperature controlled with a vessel heat exchanger 57 and aliquid temperature sensor 68 measuring either the wall temperature ofthe vessel 52, or, preferably, the temperature of the liquid 56 withinthe vessel 52. The liquid 56 continues to flow into the vessel 52 untilthe wafer 50 is completely immersed. The liquid inlet 61 is preferablyat or near the bottom of the vessel.

Pressurized gas is then delivered to the vessel 52 by opening of a gassupply valve 74, which allows gas to flow from a pressurized gas supply70, through a pressure regulator 71 and a gas filter 72, and into thevessel 52. The gas 59 supplied to the vessel 52 may be optionallytemperature controlled by a gas supply heat exchanger 76 and a gastemperature sensor 78 located downstream of the gas supply heatexchanger 76. Delivery of the gas 59 into the vessel 52 pressurizes thevessel 52, thereby increasing the concentration of gas 59 dissolved intothe liquid 56 at the gas-liquid interface 53, creating a surface tensiongradient. Gas 59 flows into the vessel until an operating pressurepreferably between 10 and 100 atmospheres, more preferably between 20and 50 atmospheres, is attained.

Once the operating pressure is reached within the vessel 52, the liquidthrottle valve 80 is opened to allow liquid 56 to begin draining out ofthe vessel 52 through the moveable drain 54. Liquid 56 is drawn into themoveable drain 54 from below to maintain the surface tension gradientalong the gas-liquid interface 53, and to prevent the gas from escapingbefore dissolving into the liquid. The flow of liquid 56 through theliquid drain valve 80 is driven by the elevated pressure within thevessel 52. The gas 59 preferably continues to flow into the vessel 52through gas supply valve 74 while the liquid 56 is being drained.

As the liquid 56 is drained, the gas-liquid interface 53 descends withinthe vessel 52. The wafer 50 is dried as the surface tension gradientpulls the liquid from the surface of the wafer 50 as the wafer 50 isexposed to the gas 59. This draining continues until the liquid leveldrops completely below the dried wafer 50. Once the wafer 50 is dried,the pressure within the vessel 52 may be reduced to ambient, by reducingdelivery of gas 59 to the vessel 52. The vessel 52 may then be openedand the dry wafer 50 removed. Any additional liquid 56 present withinthe vessel 52 may be removed via a gravity drain 81 by opening a gravitydrain valve 82 along the bottom of the vessel 52 in preparation for thedrying of additional wafers.

FIG. 3 illustrates the basic steps of a high pressure semiconductorwafer processing method. While the term “processing” here may includewafer drying, it also includes etching, cleaning, rinsing, and othertreatment steps.

Referring now in detail to FIG. 3, at least one wafer is placed into anopen vessel, as represented by step 110. The vessel is then closed witha pressure-tight lid or door, to contain elevated pressures within thevessel, as represented by step 112. Following the sealing of the vessel,a liquid is delivered into the vessel until the wafer is immersed, sothat the liquid level is above the wafer, as represented by step 114.This liquid may or may not be reactive with the wafer material,depending on the desired processing result. Upon immersion of the wafer,a pressurized gas is delivered into the vessel, as represented by step116. The particular gas to be used, as well as whether the gas isdelivered above the liquid or injected directly into the liquid, dependson the particular chemical process desired. Gases such as carbondioxide, argon, fluouromethane, and trifluoromethane may be used if aninert gas is desired, such as where the processing will include a finaldrying step. If the gas is to be used for drying, then it is importantnot to inject the gas into the liquid, to avoid bubbling and liquidcarryover. If a reactive gas is desired, then a variety of gasesincluding ozone, HCl, HF, or gaseous ammonia may be used. Operation atelevated pressures allows the gas to become dissolved in the liquid atelevated concentration levels.

The liquid may then be drained from the vessel until the liquid level isbelow the wafer, as represented by step 118. If it is not desirable ornecessary to maintain a high dissolved gas concentration at the surfaceof the liquid, then the liquid may be drained from a fixed drain alongthe bottom of the vessel. But where it is desirable to treat the surfaceof the wafer with a liquid having a high concentration of dissolved gas,such as in instances where wafer drying is desirable, then the liquidmay be drained from the vessel by way of a moveable drain positionedalong the gas-liquid interface within the vessel. The steps of immersingthe wafer with liquid, pressurizing the vessel with gas, and thendraining away the liquid, may be repeated and performed sequentiallywith different gases and liquids to accomplish several processingobjectives within the same vessel. Typically, the final wet processingstep includes immersion in deionized water and drying with an inert gas.Once the desired processing is complete, the gas pressure within thevessel is reduced, such as by reducing the flow of gas into the vessel,as represented by step 120. At that time, the vessel may be opened andthe wafer removed, as represented by step 122.

Among the benefits of wet processing wafers at high pressures is theability to safely sustain chemical treatment at high temperatureswithout approaching the boiling point of the underlying liquid. This isparticularly important where volatile and/or flammable liquids are beingused. Using the processing methods of the present invention, largeamounts of thermal energy can be made available to support reactionsthat were heretofore either too dangerous or too slow to be feasible forcommercial wafer processing. Moreover, processing under pressureconditions may also allow for a transition from a wetted state to a drystate under a gas blanket such as carbon dioxide.

FIG. 4 is a schematic diagram of a high pressure wafer processing systemwhich may be used to carry out the steps illustrated in FIG. 3. At leastone wafer 150 is placed within a process vessel 152 suitable forcontaining elevated pressures. The process vessel 152, which ispreferably emptied of any liquid before the wafer 150 is placed insideso as to flush away any contaminants, is then closed around the wafer150. The process vessel 152 preferably contains a moveable drain 154that maintains a extraction depth just below the liquid-gas interface153 within the vessel 152, such as by floating on the surface of theliquid 156 within the vessel 152 and draining liquid 156 from the bottom155 of the moveable drain 154. A moveable drain 154 is preferred whereit is desirable to treat or dry the surface of the wafer 150 with aliquid having a high concentration of dissolved gas. A gravity drain 140and gravity drain valve 141 are preferably also provided to drain anyliquid 156 remaining within the vessel 152 following the processing ofone wafer 150 to prepare for another.

Once the process vessel 152 is closed around the wafer 150, a liquidsupply valve 164 opens to allow liquid to flow from at least one sourceinto the vessel 152. The liquid 156 continues to flow into the vessel152 until the wafer 150 is completely immersed. Liquid may be suppliedvia a liquid supply pump 170 through a pressure regulator 171 and aliquid supply filter 172. Preferably, however, liquid may be suppliedvia one or more holding tanks 180, 185. Multiple holding tanks aredesirable to accomplish delivery from one holding vessel to the processvessel 152 while the other vessel is vented and re-filled with liquid.This continuous liquid delivery to the process vessel 152 may beaccomplished without the need for venting to atmosphere or utilizinghigh-pressure pumps with corresponding sealing and/or contaminationproblems. The holding tanks 180, 185 are furnished with liquid such aswater from a liquid supply 190 by way of a liquid filter 192 and liquidsupply valves 181, 186. Chemicals may be injected into the holding tanks180, 185 from chemical supplies 182, 187 through chemical supply valves183, 188 to yield mixtures such as APM, HPM, SPM, SOM, or numerous otherchemistries known in semiconductor processing. The holding tanks 180,185 may be pressurized with gas supplied from a pressurized gasreservoir 200 through a pressure regulator 202, gas filter 203, andsupply valves 204, 206. Once pressurized, the liquid may flow fromholding tanks 180, 185 to the vessel 152 by way of tank outlet valves184, 189. The liquid supplied to the vessel 152 by whatever source maybe optionally temperature controlled by way of a liquid supply heatexchanger 176 and a liquid temperature sensor 178 located downstream ofthe liquid supply heat exchanger 176. Moreover, the vessel itself may beoptionally temperature controlled by way of a vessel heat exchanger 157and a liquid temperature sensor 168 measuring either the walltemperature of the vessel 152 or, preferably, the temperature of theliquid 156 within the vessel 152.

Upon immersion of the wafer 150, pressurized gas is delivered into thevessel 152. Gases such as carbon dioxide, argon, fluouromethane, andtrifluoromethane may be used if an inert gas is desired, such as wherethe processing will include a final drying step. Whether the gas isdelivered above the liquid or injected directly into the liquid dependson the particular chemical process desired.

However, if the gas is to be used for drying, then it is preferred notto inject the gas into the liquid. If a reactive gas is desired, then avariety of gases including ozone, HCl, or gaseous ammonia may beadvantageously used. A reactive gas, however, is not desired for use inpressurizing holding tanks 180, 185. In cases where holding tanks areused to deliver liquid to the vessel 152, and reactive gases are used inthe vessel 152, then a separate inert pressurized gas supply (not shown)should be maintained for pressurizing the holding tanks. Gas 159 issupplied to the vessel 152 from a pressurized gas reservoir 200 througha gas pressure regulator 212, a gas filter 213, and a gas supply valve216. The gas 159 supplied to the vessel 152 may be optionallytemperature controlled by way of a gas supply heat exchanger 218 and agas temperature sensor 220 located downstream of the gas supply heatexchanger 218. Heating the gas may reduce the surface tension of theliquid 156 within the vessel when the gas 159 becomes dissolved in theliquid 156.

The operating pressure within the vessel is preferably maintainedbetween 10 and 100 atmospheres, more preferably between 20 and 50atmospheres. Flow rates of gas between 1-10liters per minute atoperating pressure are preferred for a vessel size of approximately 40liters. While the wafer 150 is immersed in liquid 156 within thechamber, an optional megasonic transducer 222 within the vessel 152 maybe used to assist in cleaning the wafer 150 with sound waves. Thepressurized liquid prevents cavitation and suppresses bubble formationfor more effective use of megasonics to enhance cleaning performance byminimizing power dissipation and increasing acoustic streaming. Once thepressure inside the vessel 152 attains operating levels, the liquid 156may be drained from the vessel. If it is not desirable or necessary tomaintain a high concentration of dissolved gas 159 at the surface of theliquid 156, then the liquid 156 may be drained from a fixed gravitydrain 140 along the bottom of the vessel. But where it is desirable totreat the surface of the wafer 150 with a liquid having a highconcentration of dissolved gas, such as in instances where wafer dryingis desirable, then the liquid 156 may be drained from the vessel 152 byway of a moveable drain 154 positioned along the gas-liquid interface153 within the vessel 152. Liquid 56 is drawn into the underside 155 ofthe moveable drain 154 so as to maintain the gas concentration andsurface tension gradient along the gas-liquid interface 153 by allowingthe surface film to be constantly being drained away, and to prevent thegas 159 from escaping before dissolving into the liquid 156. The flow ofliquid 156 through the drain throttle valve 180 is induced by theelevated pressure within the vessel 152, and gas 159 preferablycontinues to flow into the vessel 152 through gas supply valve 174 whilethe liquid 156 is being drained.

The steps of immersing the wafer 150 with liquid 156, pressurizing thevessel 152 with gas 159, then draining away the liquid 156 may berepeated and/or performed sequentially with different gases and liquidsto accomplish several processing objectives within the same vessel.Cycling, that is, repeatedly elevating and decreasing, the pressurewithin the vessel 152 may assist in promoting the introduction ofprocess fluids into complex wafer geometries. Once the desiredprocessing is complete, the gas pressure within the vessel is reduced,such as by reducing the flow of gas 159 into the vessel 152. At thattime, the vessel 152 may be opened and the wafer 150 removed.

FIG. 5 illustrates the basic steps of a high pressure semiconductorwafer processing method, specifically including a megasonic cleaningstep. This method may also be performed with equipment illustrated inFIG. 4. A wafer is placed into an open vessel, as represented in step250. A liquid, such as deionized water, for example, is delivered intothe vessel to immerse the wafer, as represented in step 252. Preferably,before the vessel is closed, the liquid delivered into the vesseloverflows at least one wall of the vessel to flush any loosecontaminants that may have been resident in the vessel or on the surfaceof the wafer before further processing. The vessel is then closed with apressure-tight lid, as represented in step 254. Following the sealing ofthe vessel, the vessel is pressurized by the delivery of a pressurizedgas into the vessel, as represented in step 256.

Once the wafer is immersed in pressurized liquid, megasonic waves may betransmitted into the liquid and against the wafer for maximum advantage,as represented in step 258. The megasonic transducer 222 on the vessel,shown in FIG. 4, provides the megasonic waves. As compared to liquids atatmospheric pressure, the pressurized liquid prevents cavitation andsuppresses bubble formation for more effective use of megasonics toenhance cleaning performance by minimizing power dissipation andincreasing acoustic streaming. Following the delivery of megasonic wavesinto the vessel, the wafer may be optionally rinsed and dried or simplydried, with drying being accomplished by draining pressurized liquidfrom the vessel with a moveable drain positioned along the gas-liquidinterface within the vessel. As the liquid is being drained, thegas-liquid interface descends within the vessel, and the wafer is driedas the surface tension gradient pulls the liquid from the surface of thewafer as the wafer is gradually emerges from the receding liquid. Asrepresented in step 260, the gas pressure is then reduced within thevessel, such as by reducing the pressurized gas supply to the vessel,and finally the vessel may be opened to permit the wafer to be removed,as represented in step 262.

FIG. 6 illustrates the basic steps of a high pressure semiconductorwafer processing method, specifically including the providing of asupercritical substance within the vessel. A wafer is placed into anopen vessel, as represented in step 280. The vessel containing the waferis closed with a pressure-tight lid, as represented in step 282. Next, asupercritical substance such as carbon dioxide, argon, trifluoromethane,or fluoromethane is provided within the vessel, as represented in step284. A substance in supercritical phase is neither a gas nor a liquid,but exhibits properties somewhat akin to both gas and liquid, havinghigh exchange rates and enhanced cleaning capabilities. While thesupercritical point varies by substance, it generally is obtained athigh temperatures and pressures. Accordingly, the provision of asupercritical substance within the vessel may be accomplished bydelivering a substance already in supercritical phase into the vessel,or by heating and/or pressurizing the substance within the vessel untilit reaches supercritical phase. Optionally, the substance may be cycledthrough the supercritical point within the vessel, and thereby throughliquid-gas phase changes, to obtain dramatically improved penetrationinto small wafer geometries and features, such as deep and narrow vias,by essentially flash evaporating the supercritical substance out ofthese geometries. As represented in step 286, the pressure within thevessel may be reduced to ambient condition after one or more cycles,such as by reducing the pressurized supply to the vessel, and finallythe vessel may be opened to permit the wafer to be removed, asrepresented in step 288.

FIGS. 7A and 7B illustrate a wafer processing system 299, wherein wafers300 are contained within a vessel 302 having a front wall 310, a rearwall 312, side walls 314, 316, a bottom wall 318, a hinged lid 304. Thelid 304 pivots open with a hinge 306 to allow the wafers 300 to beinserted into and removed from the vessel 302. A liquid supply orifice320, preferably mounted along or adjacent to the bottom wall of thevessel, provides a location for liquid 322 to be supplied into thevessel 302. Preferably, various chemical mixtures and rinsing liquidsmay be supplied to the vessel 302 through the liquid supply orifice 320and directed by external piping.

Liquid 322 supplied to the vessel 302 preferably immerses the wafers 300completely. When rinsing liquid is used, it continues to overflow thevessel 302 so as to flush away any loose contaminants within the vessel302 or along the surfaces of the wafers 300. The front wall 310 of thevessel 302 is shorter than the rear wall 312 to permit, when the hingedlid is open, liquid to overflow the front wall 310 into the overflowbasin 330. Overflow liquid 331 is removed from the basin 330 by a drainport 332. Limiting the height of the liquid 322 within the vessel 302also prevents liquid 322 from contacting the gas delivery orifice 308located in the lid 304, so as to avoid problems with bubbling and liquidcarryover in case drying will be performed within the vessel 302. Thelid 304 further has a protruding front wall portion 305 to mate with thereduced-height front wall 310. After liquid 322 has been delivered tothe vessel 302, pressurized gas 324 may be supplied into and pressurizethe vessel 302 through the gas delivery orifice 308.

Within the vessel 302, the wafers 300 are elevated by a pedestal 340relative to the bottom wall 318. Elevating the wafer allows the liquid322 within the vessel 302 to be drained to a level below the wafers 300before the wafers 300 are extracted from the vessel. The wafers 300 aresupported from above and below by longitudinal combs 342, which arelinked by detachable comb links 344 to maintain the position of thewafers 300 within the vessel 302. A plurality of wafers may be processedsimultaneously within a suitably configured vessel.

A floating drain ring 350 surrounds the wafers 300 within the vessel302. The floating drain ring 350 floats atop the liquid 322 within thevessel 302. The vessel 302 is illustrated as being approximately halffull of liquid 322, with the drain ring 320 floating along the liquidsurface 323. Because it floats, the drain ring 350 moves vertically withthe liquid level 323 inside the vessel 302. The drain ring 350 has aplurality of orifices or slots 352 along the underside 355 of the ring350 to drain liquid 322 from the vessel 302. It is important to drainthe vessel 302 just below the liquid surface 323 so as to constantlydrain away the surface layer of liquid 322, thereby maintaining thesurface tension gradient along the gas-liquid interface, and alsopreventing the gas 324 from escaping directly before dissolving intosurface of the liquid 322 within the vessel 302. To similarly promoteconstant and even draining away of the surface layer of liquid 322, itis preferred to have the drain ring 350 extend completely around thewafers 350.

Liquid 322 is drained from the vessel 302 through the drain ring 350 andinto the liquid outlet 356 via flexible tubing 354. Flow through thedrain ring 350, flexible tubing 354, and the vessel liquid outlet 356 ismodulated by an external throttling valve (not shown). The motive forcefor this flow is the elevated pressure within the vessel 302, and gas324 preferably continues to flow into the vessel 302 through gas supplyorifice 308 while the liquid 322 is being drained. Because the flow isdriven by a difference in pressure, rather than mere gravity, the liquidoutlet 356 need not be positioned below the drain ring 350 at all times.This permits the liquid outlet 356 to be positioned along the midpointof the front wall 310 so as to minimize the length of the flexibletubing 354. The flow rate must be adequate to create a reasonable drainrate, although one to ten liters per minute at an operating pressure isa preferred flow rate for a vessel approximately forty liters in size.

The elevated pressure within the vessel causes gas 324 to dissolve intothe liquid 322 along the gas-liquid interface 323, thus generating asurface tension gradient along the liquid surface. As the liquid 322 isbeing drained, the gas-liquid interface 323 descends within the vessel302, and the wafers 300 are dried as the surface tension gradient pullsthe liquid from the surfaces of the wafers 300 as the wafers 300 areexposed to the gas 324. This draining continues until the liquid level323 drops completely below the dried wafers 300. When processing ordrying is completed, the pressure may be reduced within the vessel 302by reducing the pressurized gas supply, and the vessel 302 may be openedto permit removal of the wafers 300. Any residual liquid 322 within thevessel 302 at the time the pressure is reduced may be drained through agravity drain orifice 358 along the bottom of the vessel 302.

The vessel 302 preferably operates at a pressure between 10 and 100atmospheres, more preferably between 20 and 50 atmospheres. The vessel302 should be constructed of a structural material such as stainlesssteel that is suitably strong to contain these elevated pressures, evenunder cycling pressure loads, with a margin for safety. Since variousliquids and gases may be used within the vessel, the surfaces of thevessel 302 contacting liquid or gas should be coated with a non-reactivesubstance such as a polymer, like polytetrafluoroethylene, or a quartzmaterial. The vessel 302 may further incorporate temperature control,such as with external heat exchangers, and megasonic transducers toprovide a wide range of processing options.

FIGS. 8A and 8B illustrate an alternative wafer processing system 399similar to the system 299 described in FIGS. 7A and 7B, but with thefollowing differences. As shown in FIGS. 8A and 8B, 15 wafers 400 arecontained within a tank 402 that is completely contained within apressurized vessel 401. The tank 402 is illustrated as being filled withliquid 422. If the tank 402 is overflowed, then any excess liquid flowsinto the surrounding vessel 401 where it can be drained through agravity drain orifice 458 at the bottom of the vessel 401. Accordingly,there is no need for an overflow basin in apparatus 399, since overflowsare contained by the vessel 401. Positioning the tank 402 within thevessel 401 simplifies locating the gas delivery orifice 408 to delivergas to the vessel 401 at a location not in contact with liquid. The gasdelivery orifice 408 is not positioned along the hinged lid 404 of thevessel 401, but rather along one stationary side 411 of the vessel.Also, since the tank 402 experiences equal pressures along all sides410, 412, and 418, the use of structural materials directly underlyingsurfaces contacting liquids and gases used for wafer processing isobviated. The system 399 otherwise operates the same as the system 299shown in FIGS. 7A and 7B. A liquid supply orifice 420, preferablymounted along or adjacent to the bottom wall 418 of the tank 402,provides a location for liquid 422 to be supplied into the vessel tank402. Within the tank 402, the wafers 400 are elevated by a pedestal 440relative to the bottom wall 418 of the tank. Surrounding the wafers 400within the tank 400 is a floating drain ring 450 that floats atop theliquid 422 within the tank 402. The drain ring 450 has a plurality oforifices or slots 452 along the underside 455 of the ring 450 to drainwater 422 from the tank 402 just below the gas-liquid interface 423.Liquid 422 is drained through the drain ring 450 and into the liquidoutlet orifice 456 from the tank via flexible tubing 454. The vessel 401preferably operates at a pressure between 10 and 100 atmospheres, morepreferably between 20 and 50 atmospheres. As the liquid 422 is beingdrained, the gas-liquid interface 423 descends within the tank 402, andthe wafers 400 are dried as the surface tension gradient pulls theliquid from the surfaces of the wafers 400. This draining continuesuntil the liquid level 423 drops completely below the dried wafers 400.

When processing or drying is completed, the pressure may be reducedwithin the vessel 402 by reducing the pressurized gas supply, and thevessel 402 may be opened to permit removal of the wafers 400. After thepressure has been reduced, any residual liquid 422 within the tank maybe drained through a gravity drain orifice 420 along the bottom of thetank.

FIGS. 9A and 9B illustrate a second alternative wafer processing system499 of the present invention. The system 499 is the same as the system399 shown in FIGS. 8A and 8B, except for differences relating todraining from the tank 502. In particular, the system 499 does not havea drain ring that surrounds the wafers 500 on all sides. Rather it has adrain bar 550 having three sides. Rather than draining liquid from thetank 502 into the liquid outlet orifice via flexible tubing, the drainbar 550 connects to the liquid outlet 556 via rigid, hollow pivotallinks 557, 559 and a hollow drain crossbar 553. Thus, the drain bar 550is hinged inside one wall 510 of the tank, and the drain bar 550 followsan arcuate path as it ascends or descends in response to changing waterlevel within the tank. A plurality of orifices 552 along the undersideof the drain bar 550 draw water from within the tank 502 just below thegas-liquid interface 423. When draining begins, liquid 522 is drawnthrough orifices 552 and into the drain bar 550. From the drain bar, theliquid 522 is travels through the hollow pivotal links 557, 559 into thehollow drain crossbar 553 and finally into the liquid outlet orifice 556to exit the tank and vessel.

A liquid supply orifice 520, preferably mounted along or adjacent to thebottom wall 518 of the tank 502, provides a location for liquid 522 tobe supplied into the tank 502 containing one or more wafers 500.Preferably, the liquid 522 immerses the wafers 500. Within the tank 502,the wafers 500 are elevated by a pedestal 540 relative to the bottomwall 518 of the tank 502. The vessel 501 preferably operates at apressure between 10 and 100 atmospheres, more preferably between 20 and50 atmospheres.

As the liquid 522 is being drained, the gas-liquid interface 523descends within the tank 502, and the wafers 500 are dried due to thesurface tension gradient created by high pressure gas dissolved into theliquid 522. The surface tension gradient at the gas-liquid interfacepulls the liquid off of the wafers 500. The drain bar 550 maintains thissurface tension gradient by removing a top layer of the liquid 522within the tank 502, so that fresh liquid can continuously come intocontact with the high pressure gas delivered into the vessel 501.

The draining continues until the liquid level 523 drops completely belowthe dried wafers 500. When processing or drying is completed, thepressure may be reduced within the vessel 502 by reducing thepressurized gas supply, and the vessel 502 may be opened to permitremoval of the wafers 500. After the pressure has been reduced, anyresidual liquid 522 within the tank may be drained through a gravitydrain orifice 520 along the bottom of the tank.

Although floating drains have been described, a non-floating drain,moved with an actuator, can also be used.

The systems described above provide the advantage that the wafers neednot be moved during processing. This eliminates mechanical sources ofcontamination. In-situ rinsing and drying may be performed. The use ofhigh pressure provides additional processing options. Boiling points aresuppressed. Higher processing temperatures can be used. Processingperformance of various process chemistries is improved. Drying can beachieved without using organic vapors, such as alcohols, therebyavoiding the disadvantages associated with such organic vapors. Thesolubility of gases in the liquids is increased. Cavitation and bubbleformation are reduced, allowing for more effective use of megasonics toenhance cleaning. Reagent penetration into small geometries is improved.

Though the present invention has been described in terms of certainpreferred embodiments, other embodiments apparent to those skilled inthe art should also be considered as within the scope of the presentinvention. Elements and steps of one embodiment may also readily be usedin other embodiments. Substitutions of steps, devices, and materials,will be apparent to those skilled in the art, and should be consideredstill to be within the spirit of the invention. Accordingly, theinvention should not be limited, except by the following claims, andtheir equivalents.

What is claimed is:
 1. A method for processing a wafer, comprising thesteps of: placing the wafer into a vessel; delivering a liquid into thevessel, creating a liquid level; sealing the vessel so that it ispressure-tight; delivering a pressurized gas into the vessel;transmitting sonic energy to the wafer; draining the liquid from thevessel; reducing the gas pressure within the vessel; and removing thewafer from the vessel.
 2. The method of claim 1, wherein the pressurizedgas is delivered into the vessel at a location above the surface of theliquid.
 3. The method of claim 1, wherein the delivery of the liquidinto the vessel continues until the liquid overflows at least one sideof the vessel before the vessel is closed with the pressure-tightclosing member.
 4. The method of claim 1, wherein the delivery ofpressurized gas continues while the liquid is drained from the vessel.5. The method of claim 1, wherein the vessel is sealed with apressure-tight closing member having an orifice, and the pressurized gasis delivered through the orifice in the closing member.
 6. The method ofclaim 1, wherein the pressurized gas is inert and not insoluble inwater.
 7. The method of claim 1, wherein the pressurized gas is selectedfrom the group consisting of carbon dioxide, argon, trifluoromethane,and fluoromethane.
 8. The method of claim 1, wherein the delivery ofpressurized gas into the vessel pressurizes the vessel to a pressurebetween 10 and 100 atmospheres.
 9. The method of claim 1, wherein thedelivery of pressurized gas into the vessel pressurizes the vessel to apressure between 20 and 50 atmospheres.
 10. The method of claim 1wherein the liquid is drained from a location just below the liquidsurface.
 11. The method of claim 1 further comprising the steps ofcontrolling the temperature of the gas.
 12. The method of claim 1further comprising the steps of controlling the temperature of theliquid.
 13. The method of claim 1 further comprising the steps ofcontrolling the temperature of the vessel.
 14. The method of claim 1,wherein: the liquid is pressurized before being delivered to the vessel;and the vessel is closed with a pressure-tight closing member before theliquid is delivered into the vessel.
 15. The method of claim 1 furthercomprising the step of stopping the delivery of pressurized gas into thevessel before draining any liquid from the vessel.
 16. The method ofclaim 1 further comprising the steps of stopping delivery of pressurizedgas into the vessel before all liquid is drained from the vessel.
 17. Amethod for processing a wafer, comprising the steps of: placing thewafer into a tank within a vessel; delivering a liquid into the tank,creating a liquid level within the tank; closing the vessel with apressure-tight closing member; delivering a pressurized gas into thevessel at a location above the liquid level; transmitting sonic energyinto the liquid; draining the liquid from the tank; reducing the gaspressure within the vessel; and removing the wafer from the tank. 18.The method of claim 17, wherein the delivery of the liquid into the tankcontinues until the liquid overflows at least one side of the tank. 19.The method of claim 17, wherein the delivery of pressurized gascontinues while the liquid is drained from the vessel.
 20. The method ofclaim 17, wherein the pressurized gas is selected from the groupconsisting of carbon dioxide, argon, trifluoromethane, andfluoromethane.
 21. The method of claim 17, wherein the delivery ofpressurized gas into the vessel pressurizes the vessel to a pressurebetween 10 and 100 atmospheres.
 22. The method of claim 21, wherein thepressurized gas is delivered into the vessel at a rate betweenapproximately 1 and 10 liters per minute.
 23. The method of claim 17,wherein the delivery of pressurized gas into the vessel pressurizes thevessel to a pressure between 20 and 50 atmospheres.
 24. The method ofclaim 17 further comprising the steps of controlling the temperature ofthe gas.
 25. The method of claim 17 further comprising the steps ofcontrolling the temperature of the liquid.
 26. The method of claim 17further comprising the steps of controlling the temperature of thevessel.
 27. The method of claim 17, wherein: the liquid is pressurizedbefore being delivered to the vessel; and the vessel is closed with apressure-tight closing member before the liquid is delivered into thevessel.
 28. A method for processing a wafer, comprising the steps of:placing the wafer into a vessel; making the vessel pressure-tight;providing a pressurized, supercritical substance within the vessel;transmitting sonic energy to the wafer; reducing the pressure within thevessel; and removing the wafer from the vessel.
 29. The method of claim28, wherein the supercritical substance is selected from the groupconsisting of carbon dioxide, argon, trifluoromethane, andfluoromethane.
 30. The method of claim 28 further comprising the step ofcontrolling the temperature of the vessel.
 31. The method of claim 30further comprising the steps of: delivering the substance into thevessel in a pressurized but sub-critical phase, and transitioning thepressurized substance from sub-critical to supercritical phase byheating the vessel.
 32. The method of claim 28, wherein thesupercritical substance is supercritical upon delivery to the vessel.33. The method of claim 28, wherein: the reduction of pressure withinthe vessel is reduced to a level below the critical point of thesubstance; and the steps of providing the supercritical substance withinthe vessel and reducing the pressure to below the critical point of thesubstance are repeated at least once before the wafer is removed fromthe vessel.
 34. A method for processing a wafer comprising the steps of:placing the wafer into a vessel; closing the vessel with apressure-tight closing member; delivering a liquid into the vessel untilthe wafer is immersed in liquid and the liquid level is above the wafer;delivering a pressurized gas into the vessel; transmitting sonic energyinto the liquid; draining the liquid from the vessel until the liquidlevel is below the wafer; reducing the pressure within the vessel; andremoving the wafer from the vessel.
 35. The method of claim 34, whereinthe pressurized gas is delivered into the vessel while the wafer isimmersed in liquid and the liquid is drained from the vessel.
 36. Themethod of claim 34, wherein the steps of immersing the wafer in liquidand draining the liquid from the vessel until the liquid level is belowthe wafer are repeated at least once.
 37. The method of claim 34 furthercomprising the step of heating the vessel.
 38. The method of claim 34,further comprising the step of heating the wafers to a temperaturegreater than 100 degrees C.
 39. The method of claim 34, wherein thevessel is pressurized between 10 and 100 atmospheres.
 40. The method ofclaim 34, wherein the vessel is pressurized between 20 and 50atmospheres.
 41. The method of claim 34, wherein the pressurized gascomprises ozone.
 42. The method of claim 34, wherein the liquid isdrained at a location just sufficiently below the liquid surface toprevent gas from escaping out of the vessel.
 43. The method of claim 34,wherein the gas is delivered into the vessel at a location above theliquid surface.
 44. The method of claim 34, wherein the gas is deliveredinto the vessel at a location below the liquid surface.
 45. The methodof claim 34 further comprising the step of controlling the temperatureof the vessel.
 46. An apparatus for processing an article comprising: apressure vessel having at least one side, a bottom, a sealable closingmember suitable for containing elevated pressures within the vessel, aliquid supply orifice, and a gas supply orifice; a liquid supply valvesuitable for delivering liquid through the liquid supply orifice tocreate a liquid level within the vessel, a gas supply valve fordelivering pressurized gas into the vessel at a location above theliquid level; a sonic transducer on the vessel for transmitting sonicenergy into the liquid; an article support in the vessel; and a movableliquid drain within the vessel.
 47. The apparatus of claim 46 furthercomprising a throttle valve for controlling flow through the movableliquid drain.
 48. The apparatus of claim 48, wherein the liquid drainfloats on the liquid within the vessel.
 49. An apparatus for processinga workpiece comprising: a vessel having a gas supply orifice and asealable lid suitable for containing elevated pressures within thevessel; a tank within the vessel, the tank having at least one side, abottom, and a liquid supply orifice; a liquid supply valve suitable fordelivering liquid through the liquid supply orifice to create a liquidlevel within the tank; a gas supply valve for delivering pressurized gasinto the vessel at a location above the liquid level; a sonic transduceron or in the vessel; a support in the tank for supporting at least oneworkpiece; and a vertically displaceable liquid drain within the tank toextract liquid at a depth just below the liquid level within the tank.50. The apparatus of claim 49, wherein the liquid draining member floatson the liquid within the vessel.
 51. The apparatus of claim 49, whereinthe vessel contains multiple chambers, with the wafer being placed in afirst chamber and liquids being pressurized in a second chamber forsubsequent delivery into the first chamber.
 52. The apparatus of claim51, wherein a valve is installed between the first chamber and thesecond chamber.
 53. The apparatus of claim 51 further including apressure bladder in the second chamber.